The stator cores of electrical generators and other electrical machines are made up of a "stack" of thousands of individual steel sheets or laminations. The thickness of an individual sheet is measured in thousandths of an inch. Each of the laminations is coated with a thin layer of electrical insulation to insulate it electrically from its neighbors. This insulation prevents the 60 HZ alternating magnetic flux in the stator core from inducing eddy currents between laminations. If the insulation between adjacent laminations becomes damaged during assembly, operation or maintenance, a conducting path may be formed through which currents are induced by the alternating flux. These conducting currents create "hot spots" which, if undetected, can result in catastrophic failure of the machine.
One type of apparatus suitable for detecting such stator core hot spots is an Electro-magnetic Core Imperfection Detector ("EL-CID"). An EL-CID detects potential hot spots electromagnetically by exciting the stator core and then measuring any resulting fault currents flowing through a damaged region. Typically, the EL-CID's sensors are held against the stator core laminations and transported the length of the stator core. As the sensors move along the length of the core, they produce an analog signal having a magnitude proportional to detected fault currents. By examining a plot of the magnitude of the analog signal versus the sensor distance along the length of the core, operators are able to detect which stator laminations have damaged insulation.
U.S. Pat. No. 4,970,890, assigned to Westinghouse Electric Corp. and incorporated herein by reference, discloses a stator core inspection system that employs such an EL-CID detector. In the past, the analog output signal from the EL-CID detector was fed directly to an X-Y plotter for visual inspection by the operator. From the plot, the operator was able to determine the location of any hot spots. In the '890 patent, however, the analog EL-CID signal is sampled by an analog-to-digital converter and the digital representation is stored in a computer. The operator is then able to use the panning and zooming capabilities of the computer to examine the EL-CID signal more closely.
One problem with all EL-CID inspection systems, and the system of the '890 patent in particular, is that the power plant surroundings and machinery introduce a significant amount of noise in the analog EL-CID signal. Noise in the EL-CID signal makes it more difficult for the operator to discover faults. Because analog-to-digital converters naturally perform some low-pass filtering at certain sampling rates, analog-to-digital conversion of the EL-CID signal eliminates some of the noise. However, this natural filtering is not enough to remove the bulk of the background noise from the EL-CID signal. Consequently, there is a need for a system and method for removing noise from an EL-CID signal while keeping the integrity of the EL-CID signal intact. The present invention satisfies this need.